Method of zonal melting of materials

ABSTRACT

A method of zonal melting of materials without a crucible comprises the following steps: placing a layer of unrefined material upon the circumferential surface of a previously molten ingot of refined material; and zonal melting of the layer and ingot together in the course of which a floating zone is embedded to a predetermined depth of the initial ingot, thereby increasing the circumferential diameter of the ingot.

United States Patent [191 Melnikov et al.

[ Nov. 20, 1973 METHOD OF ZONAL MELTING OF MATERIALS [76] Inventors: Mikhail Vasilievich Melnikov, ul.

Akademika Koroleva, 5, kv. 221; Aleko Konstantinovich Kakabadze, Kaliningrad Moskovskaya obl., ul. Komitetskaya, 3, kv. 6, both of Moscow, U.S.S.R.

{22] Filed: Apr. 3, 1968 [21] Appl. No.: 718,431

3,490,961 l/197O Frieser 117/933 1,703,658 2/1929 Coles 75/65 R 2,719,799 10/1955 Christian.. 75/65 ZM 2,805,148 9/1957 DeLong 75/10 R 2,870,006 1/1959 Morning... 75/10 R 2,753,254 7/1956 Rick 75/63 X 3,179,593 4/1965 ReuscheL. 148/1.6 X

3,239,899 3/1966 Johnson 75/63 X 2,805,148 9/1957 DeLong ..75/65 X FOREIGN PATENTS OR APPLICATIONS 876,467 9/1961 Great Britain 75/10 375,304 6/1932 Great Britain 75/10 R 654,763 l/l963 Italy 75/65 ZM 39/22499 3/1960 Japan 148/1.6

Primary Examiner-L. Dewayne Rutledge Assistant Examiner-Peter D. Rosenberg Att0rneyWaters, Roditi, Schwartz & Nissen 57 ABSTRACT A method of zonal melting of materials without a crucible comprises the following steps: placing a layer of unrefined material upon the circumferential surface of a previously molten ingot of refined material; and zonal melting of the layer and ingot together in the course of which a floating zone is embedded to a predetermined depth of the initial ingot, thereby increasing the circumferential diameter of the ingot.

5 Claims, 2 Drawing Figures F/GZ METHOD OF ZONAL MELTING OF MATERIALS The present invention relates to methods of zonal melting without a crucible preferably used for refining materials and obtaining monocrystalls.

A method of zonal melting without a crucible used for refining materials and obtaining monocrystals proves ideal as far as sterility of the process is concerned. Therefore it is most frequently used in melting materials and semi-conductors, which interact with the materials of a container.

However, notwithstanding its advantages this method does not ensure high efficiency. Apart from the fact that only one zone can be created by this method, an ingot exposed to zonal melting may have but a limited cross-section, e.g., a tungsten rod can have a maximum diameter of to 12 mm., determined by the stability of the zone, whose width in its turn, depends upon the value of the surface tension and the hydrostatic pressure created by the column of the molten material.

It was established that for rods of great radius the maximum length of the zone in the vertical arrangement of zonal melting without a crucible is determined by the following equation:

l= 2.84 (aldq) one-half where l is the length of the zone,

0' is the surface tension,

d is the specific gravity of the melt,

g is the gravity acceleration.

Thus, the formula (0'/d) one-half serves as a criterion of the stability of the zone (W. Heywang, Zeitschrift Naturforschung, lla, 238, 1956).

However, in melting samples with enlarged crosssections it is practically difficult, if not impossible, to conduct heating so as not to exceed the zone margin. Consequently, a deformation and dripping of the molten zone take place.

Known in the art are methods of enhancing the efficiency of zonal melting without a crucible.

Such is the use of small-size ingots in the form of a sheet or a tube which allows increasing increase the cross-section of the material being refined under zonal melting without a crucible. ln this case the stability of the zone is not impaired and the efficiency is enhanced through the increase of sheet width or tube diameter.

lngots with square section are also used. During the melting process, due to the heat radiation, the ribs of the ingot do not melt and thus ensure an additional maintenance of the molten zone.

Also known is a method of enlarging the diameter of I the ingot subjected to zonal melting by maintaining the molten zone by molybdenum pins protected by ceramic hoods placed along the periphery of the sample.

Methods are also known which employ electrodynamic (magnetic) maintenance of the floating zone based on the interaction of the inducing and the induced currents under high-frequency heating. Thus, in zonal melting of copper, which has small surface tension and considerable specific gravity, a low-frequency conic inductor similar to that employed for melting metals in suspension, was used.

Another version of this method, electromagnetic maintenance of the zone, is based upon the interaction of electric and magnetic fields. According to this method direct current is passed through an ingot being refined while a section of the zone is placed within the field of a permanent magnet. The force that pushes out the conductor supports the molten section and allows enhancing the efficiency of the method considerably or are applicable for a limited number of materials, preferably with low and medium melting temperatures.

An object of the present invention is to eliminate the disadvantages inherent in zonal melting without a crucible, and to provide a method allowing a considerable increase of the cross-section of ingots.

This object is attained through the application of a method of zonal melting without a crucible, in which, according to the invention, a layer of the material which is being refined, is placed upon the surface of the previously molten ingot and a zonal melting is performed, in the course of which a floating zone is embedded to a certain depth within the initial ingot.

It is expedient to repeat the process of applying the layer of the refined material and zonal melting with the same ingot several times in order to obtain the necessary cross-section.

Melting of the ingot should be conducted in several zones in order to raise the efficiency.

To obtain a monocrystalline ingot the initial ingot should be a monocrystal.

The implementation of said method results in increasing the cross-section of the molten ingots by several dozen times and raising the efficiency in obtaining refined material.

Below the invention is illustrated by an exemplary vertical zonal melting, without crucible, of ingots with cylindrical section and with reference to the accompanying drawings in which:

FIG. 1 is a cross-section of the ingot in the region of the floating zone; and

FIG. 2 diagrammatically illustrates the method of conducting multizonal melting.

For increasing the section of the ingot by the method of the invention, an ingot 1 is taken, molten earlier by the conventional method of melting without a crucible (FIG. 1), a layer of unrefined material 2 is placed upon its surface and a zonal melting of the layer 2 is conducted by moving the zone 3 along the two-ply ingot l in the direction indicated by the arrow. The floating zone 3, crossing the surface 5 of the initial ingot 1 is embedded into it, to a certain depth, and forms a new border 4. To balance the composition of the basic ingot and the unrefined layer a zonal melting of the latter is repeated several times.

After a single process is carried out it can be repeated in the same sequence: a layer of unrefined material is placed upon the ingot with a larger cross-section and a and zonal melting is repeated. Thus, by repetition of the process for the required number of times an ingot with the necessary cross-section can be obtained.

In order to secure a most efficient increase of the cross-section of the ingot at the initial stage of implementation of the method, it is expedient to take an original ingot l with a maximum diameter Do. Monocrystalline ingots are obtained only when the original ingot 1 is a monocrystal. Being seeded by it the layer 2 will have the same orientation as a result of the directed crystallization.

It should be noted that the maximum width of the layer 2 of the unrefined material will be determined by the same criterion of stability of the zone (CT/d) onehalf as in case of a single zonal melting without a crucible.

However, due to the fact that the ingot 1 performs the role of a one-sided support of the floating zone 3,

the size of the ultimate width of the zone 1 will be somewhat greater than in the zonal melting of the original ingot without a crucible. Therefore each time the method is implemented the diameter of the ingot can be increased by the size of the maximum diameter of the original ingot molten by means of a vertical zonal melting without a crucible.

The layer 2 can be put upon the ingot l by means of known techniques, as by pressing on of finely dispersed powder, winding, vacuum spraying, electrolytic deposition, etc.

The present method has another advantage, apart from those mentioned above. Since melting is not a through one but superficial, zonal melting can be performed simultaneously in several points of the ingot as is the case with the conventional container method of zonal melting.

We shall next consider the arrangement of multizonal melting of ingots without containers effected by this method as effected in two-zone vertical melting of cylinder ingots without a crucible.

A layer of unrefined material 2 is placed upon the previously refined ingot 1 (FIG. 2) and zonal refining is carried out by moving the zone 3 along the two-ply ingot in the direction indicated by the arrow. At a certain instant melting of the second zone 4 is conducted simultaneously with the melting of the first zone; these processes take place in parallel along the total length of the ingot. Embeddings 5 and 6 of the molten zones 3 and 4 into the surface of the original ingot 1 can be similar or different in length. In the case being considered, the zone 4 embraces a somewhat greater depth of the original ingot 1 than does the zone 3.

in view of the fact that in two-zone melting refining takes place simultaneously in two sections, refining will be as efficient as in two passes of one zone.

The above said method of multizonal melting is equally applicable for refining and obtaining monocrystals.

Thus, through a layer-per-layer gradual build-up of the cross-section of an ingot, its size can be increased considerably which depends basically upon the parameters of the melting installation.

It should be mentioned that multizonal melting not only increases the efficiency of zonal melting without a crucible but also allows adjusting the distribution of temperature along the ingot during the melting process.

What is claimed is:

l. A method of zonal melting of materials without a crucible comprising providing a molten ingot of refined material having an exposed circumferential surface, applying onto the exposed circumferential surface of said previously molten ingot of refined material a layer of the same material thereas but unrefined, and subjecting the layer and ingot to zonal melting in the course of which a floating zone is established extending to a predetermined depth into the initial ingot thereby to buildup the circumferential cross-section of the ingot.

2. A method as claimed in claim 1, wherein the applying of the layer of unrefined material and of conducting zonal melting of the layer are repeated several times for the same ingot.

3. A method as claimed in claim 1, wherein the zonal melting is effected simultaneously in a plurality of zones.

4. A method as claimed in claim 3, wherein the depth of the floating zones in the zonal melting zones is different.

5. A method as claimed in claim 1, wherein the initial ingot is constituted as a monocrystal and is used to obtain monocrystalline ingots. 

2. A method as claimed in claim 1, wherein the applying of the layer of unrefined material and of conducting zonal melting of the layer are repeated several times for the same ingot.
 3. A method as claimed in claim 1, wherein the zonal melting is effected simultaneously in a plurality of zones.
 4. A method as claimed in claim 3, wherein the depth of the floating zones in the zonal melting zones is different.
 5. A method as claimed in claim 1, wherein the initial ingot is constituted as a monocrystal and is usEd to obtain monocrystalline ingots. 